In many environmental monitoring applications (such as forest monitoring, soil moisture measurement, ground water quality monitoring) sensors are sparsely deployed to over a wide area, e.g., hundreds of meters away from each other. The data sensed by the sensors is collected at a “sink”, which may be one of the sensors. The other sensors transmit the data they sense to the sink by wireless transmissions. Thus, the sensors form a wireless communication network in which the sensors are respective nodes, also referred to in this document as “motes”. Not all the sensor nodes transmit the data directly to the sink; instead, information can be relayed from one sensor node to another until it reaches the sink.
Unfortunately, traditional wireless sensor networks are not designed for a sparse network application such as environmental monitoring. Instead, they are designed for dense networks with sensor motes spaced apart by short communication distances. If conventional network communication techniques are used in a sparse network application, there is significant deployment waste, as many sensor nodes do not contribute to sensing data but just to maintaining the network connectivity.
It is viable to apply technologies like WiMAX and cellular communication to achieve long-distance communications. However, the power consumption of WiMAX (about 200 mW) and cellular modules (typically 500 mW transmission power) is too high for typical sensor motes powered by batteries (about 54 mW). If a commercial cellular communication network is used, extra data cost may be incurred. Other hardware aided solutions, e.g., using high transmission power, or special hardware like high gain or directional antennas, are typically not applicable. Power consumption is a major consideration. Excessively higher power will be incurred to ensure communication quality over longer distances. In many places, such high transmission power in the ISM band is prohibited. For example, the maximum transmission power of 868 MHz is limited to 25 mW (14 dBm) for outdoor use in Singapore and Europe. Furthermore, those solutions add additional hardware overhead and impair the generality, e.g., most general MAC (media access control) and routing approaches are based on omnidirectional antennas and cannot be applied to directional antennas.
Some low-power sensor devices have been developed for long-distance communication, such as TinyNode [11] and Fleck-3 [8]. They provide long communication distance with low data rates. For instance, TinyNode adopts the Semtech XE1205 RF radio that increases the receiver sensitivity by a built-in low-noise amplifier and a baseband amplifier. TinyNode is able to achieve a theoretical communication distance up to 1.8 km by lowering the bit rate to 1.2 kb/s. Those long-distance devices provide the opportunity of building a sparse sensor network across large areas.